[lldb, lldm-mi] Fix hanging of -exec-run command.

Summary: -exec-run command hanged in case of invalid or dummy target.

Reviewers: aprantl, stella.stamenova

Reviewed By: aprantl

Subscribers: ki.stfu, llvm-commits, lldb-commits

Differential Revision: https://reviews.llvm.org/D47678

llvm-svn: 333844